1. Field of the Invention
Aspects of the present invention relate to a thin film transistor (TFT), a method of fabricating the same, and an organic light emitting diode (OLED) display device including the TFT.
2. Description of the Related Art
In general, a polycrystalline silicon (poly-Si) layer is widely used as a semiconductor layer for a thin film transistor (TFT), because a poly-Si layer has a high field-effect mobility and may be applied to a high-speed operating circuit, or to a complementary-metal-oxide-semiconductor (CMOS) circuit. A TFT using a poly-Si layer is typically used as an active device of an active-matrix liquid crystal display (AMLCD), or as a switching device or a driving device of an active-matrix organic light emitting diode display device (AMOLED).
The crystallization of an amorphous silicon (a-Si) semiconductor layer may be performed using a solid phase crystallization (SPC) process, an excimer laser annealing (ELA) process, a metal induced crystallization (MIC) process, or a metal induced lateral crystallization (MILC) process. Specifically, the SPC process includes annealing an a-Si layer for several hours, to several tens of hours, at temperatures below 700° C., which is the temperature at which a glass substrate, used in a TFT for a display device, is deformed. The ELA process includes heating an a-Si layer to a high temperature, in a short amount of time, by irradiating the a-Si layer with an excimer laser. The MIC process includes bringing a metal, such as nickel (Ni), palladium (Pd), gold (Au), or aluminum (Al), into contact with an a-Si layer, or injecting the metal into the a-Si layer, to phase change the a-Si layer into a poly-Si layer. Also, the MILC process includes sequentially inducing the crystallization of an a-Si layer, while laterally diffusing a silicide, which is obtained by a reaction between a metal and silicon.
However, since the SPC process involves annealing a substrate at a high temperature, for a long period of time, the substrate can be easily damaged. Also, the ELA process requires an expensive laser apparatus and may damage the crystallized surface, thereby degrading an interface between a semiconductor layer and a gate insulating layer.
A vast amount of research has been conducted on methods of crystallizing an a-Si layer using a metal, because such methods allow an a-Si layer to be crystallized at a lower temperature, for a shorter amount of time, than the SPC method. Typical methods of crystallizing an a-Si layer using a metal include an MIC method, an MILC method, and a super grain silicon (SGS) crystallization method. In these methods, however, the device characteristics of a TFT may be degraded, due to contamination caused by a crystallization-inducing metal. Accordingly, after an a-Si layer is crystallized using the crystallization-inducing metal, a gettering process is performed to remove the remaining crystallization-inducing metal.
A gettering site is formed in a semiconductor layer, in order to perform the gettering process. Conventionally, the formation of the gettering site involves implanting a gettering material into a region of the semiconductor layer, using a contact hole, which is used to connect source and drain regions of the semiconductor layer with source and drain electrodes. However, when a distance between a channel region and the contact hole is increased, a gettering process that uses only the contact hole gettering site is not very effective.